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Some questions about telescopes

Tzitzis

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Tzitzis

Tzitzis

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Well I'm completely new to this forum and relativelly new to astronomy. I usually stargaze with my binoculars and I feel I can find my way in the night sky. SO I'm thinking of getting a telescope. I started looking scopes, values and stuff. So I have some questions:

1. What is focal length and focal ratio?

2.Spherical and parabollized mirrors (what's the difference, which is better or more useful)?

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Pondus

Pondus

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Focal length  distance between eyepiece (or more correctly point of focus) and mirror, or lens. Might not be a strictly correct definition, but close enough to roughly understand what it means.

Focal ratio     : Focal length / aperture    1200mm focal length with aperture 8" or 200mm gives F/6

Focal ratio defines how 'fast' a scope is. The lower ratio the faster the scope is ( me thinks  :smiley: ).

The thread  'Abbreviations, Acronyms, and Glossary of Terms' is very helpful regarding subjects like this.  

Rune

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Pondus

Pondus

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Should I delete the post?

No you should not  :smiley: . A lot of people (definitely including me, a novice indeed) learn alot from these kind of of questions. I guess thats is why "Getting started general help and advice" exist.

Rune

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Scott

Scott

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Should I delete the post?

goodness no... Theres nothing wrong with starting a new thread, it will be more suited to your needs. Generally speaking, parabolic mirrors are superior to spherical ones.in fact, most newer scopes from the better brands are spherical. I'm not the one to explain the science behind it, I'll leave that for someone more clever than me :D.

For purely visual work,the faster your scope (smaller f/number) the better your eyepices will need to be. unfortunately, when you start to get into the bigger apeture the lesslikely you are to have a slower scope (it will become too long to handle).

Another concideration will be the type of mount. for visual, many will suggest a dobsonian mounted newt. (me included) for photography, an equitorial mount is almost essential (for dso's).

Sorry if this is confusing. any questions, just ask :D

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Tzitzis

Tzitzis

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I do not own a telescope. I searched a bit. A few days back i found this which caught my attention:

http://eu.telescope.com/Telescopes/Reflector-Telescopes/Reflector-Telescopes-with-Equatorial-Mounts/Orion-SpaceProbe-130-EQ-Reflector-Telescope/pc/488/c/497/sc/528/p/103621.uts

I thought I buy a lunar filter and a barlow 2x and I make it a great starter scope. That's only a thought, my thought, meaning I don't have any suggestions or advice from other people. I saw that it has a spherical mirror and I was wandering if that is a problem in an apperture of 130mm and a f/6.9 (Fl=900mm). I would like to get the scope and I'm interested in DSO's (keep in mind I have never gazed through a scope before). Is this a good scope for me? I'm not waiting to find all the Messier objects or watch any of them with a decent view, but it's in my budget with EQ mount. 

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Stu

Stu

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Parabolic mirrors are better. Spherical mirrors have spherical abberation which means they can focus all areas of the mirror to the same point. The parabolic mirror should give you better and sharper images

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knobby

knobby

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How about this one:

http://www.firstlightoptics.com/reflectors/skywatcher-explorer-150p-eq3-2.html

It has parabolic mirror, barlow 2x, eq mount. I will get a moon filter and it will be around 300 Euros.

I think a dob is not a very good idea cause I will need to move it a lot and dobs arent's so easily mavable...I think xD. 

Yep, thats a good one if you can find one in Greece

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Cornelius Varley

Cornelius Varley

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Doesn't the Skywatcher 130P have a parabolic mirror for similar money?

http://www.firstlightoptics.com/reflectors/skywatcher-explorer-130p.html

The Spaceprobe 130/900 is the same as the Skywatcher 130/900 which has a spherical mirror. At this aperture there is little difference in performance between a spherical and parabolic mirror.

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Stu

Stu

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The Spaceprobe 130/900 is the same as the Skywatcher 130/900 which has a spherical mirror. At this aperture there is little difference in performance between a spherical and parabolic mirror.

I thought the 130P was parabolic? That's what info on the web says...

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Dave In Vermont

Dave In Vermont

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The faster a scope's optics are, the less time it will take to take a good photo/image through it. Here's an example: If you have a scope that is F/10, and let's say it  takes 10 seconds to take a photo/image through it, if you have a scope with F/4 optics, it will take 4 seconds to get the same image through it. This is very useful if you have the scope mounted on a mount that doesn't track very accurately. The less time it takes, the less the scope will drift off target during the exposure.

However, the faster scope will also give you lower magnification with a given eyepiece:

Let's say you have a scope with a 100mm aperture, and it's a F/10 scope. This translates to 100 X 10 = 1,000. So 1,000 is your Focal-Length. To find the magnification of, say, a 10mm Eyepiece you go: 1,000 / 10mm = 100X Magnification.....Now take a scope, also with an aperture of 100mm, but it's an F/4. And your 10mm Eyepiece (EP). We go: 100 X 4 = 400mm Focal-Length (F.L.) And your magnification is: 400 / 10 = 40X Magnification.

Get it? Good Monkey! :grin::eek::grin:

Enjoy!

Dave

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John

John

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How about this one:

http://www.firstlightoptics.com/reflectors/skywatcher-explorer-150p-eq3-2.html

It has parabolic mirror, barlow 2x, eq mount. I will get a moon filter and it will be around 300 Euros.

I think a dob is not a very good idea cause I will need to move it a lot and dobs arent's so easily mavable...I think xD. 

Dobs are easy to move and to set up. Easier to use than equatorials if you are only interested in visual astronomy. I can set my 12" (30cm) dobsonian up in around 1-2 minutes. I can move it easily from the house to the garden in 2 sections which fit together in seconds.

I would not want to carry a dob over 200mm in aperture down stairs though but the same would go for an equatorially mounted scope.

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jimbo747

jimbo747

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Now I get it. So if you are not interested in astrophotography, you'd better get the bigger focal lenght.

Thanks Dave!

Not necessarily! Longer fl are great for planets, or close ups of the moon, but if you want to view the pleides, or orion or any larger dso on all ots glory you might struggle. Pleides look far better in my 10x50 binoculars than my 1500fl maksutov with 25mm ep

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